This invention relates to fittings for pressurized fluid lines, such as gas lines and the like, especially to fittings for natural gas and/or propane lines, for example fittings of the type commonly called "pigtail" fittings.
Pigtail fittings are a type of fitting used in the high pressure side of household, recreational, and light industrial gas lines, etc. For instance, a pigtail fitting is used to connect a propane tank valve to a regulator. A pigtail fitting usually has an elongated tube of soft metal (e.g., copper tubing) with at least one nipple-type fitting secured to an end thereof. The nipple has a bore therethrough which receives the tube. A left-hand-threaded nut is also provided to tighten the nipple into a matingly-configured fitting or receptacle on the tank valve or regulator. The dimensions and general nature of such fittings are standardized throughout the industry.
There a flammable gas, such as natural gas, is to be carried in the lines, it is critical that the fittings not only join together in a tight fashion but are also secured to the gas tubing such that the gas cannot leak around the fittings.
ln order to accomplish such non-leaking connections, it has heretofore been deemed necessary to secure the fittings to the tubing by soldering them. This has been particularly the case with pigtail connectors because the elongated tube on which the fittings are mounted is usually curved, shaped, or looped (giving rise to the name "pigtail". If the nipple is not secured tightly to the tube, the nipple may twist on the tube when the tube is bent, ruining the gas-tight seal between the nipple and the tube, soldering has heretofore been the only practical way to secure the nipple to the tube to create a strong enough seal to resist such twisting.
Soldering fittings onto tubing is an onerous and time-consuming process, requiring that the surfaces to be soldered are first thoroughly cleaned, applying flux, joining the parts to be soldered, and heating the parts while carefully and accurately applying the solder to the joints. Such procedures are inevitably expensive when carried out as part of commercial manufacturing operations. This is true whether done by hand or by machine, since while machines can be used to heat and solder the parts automatically, such machines are expensive and require considerable manual feeding of parts, etc. Furthermore, if done either by hand or by machine, soldering is an energy-intensive and, therefore, costly process.
Soldering processes also require that the soldered assembly be cleaned after manufacture and prior to sale, because the soldering process scorches and discolors the product. Typically, such cleaning involves use of an acid both containing a strong acid, such as chromic acid. Such acids pose disposal problems inasmuch as they cannot be dumped into sewers, injected into the ground, or introduced into watercourses. Acid cleaning also requires expensive machinery to be an efficient operation. Such equipment contributes greatly to the cost of the finished article.
There has been at least one proposed alternative to soldering fittings of the type under consideration. This alternative proposed a special pigtail fitting having a nipple whose bore, which telescopically receives the tube, has several spaced, parallel, annular grooves. The tubing, made from a soft material, such as copper, is inserted into the bore and swaged outwardly against the annular grooves by a removable swaging tool, such that the outside surface of the tube is made to conform tightly to the grooved configuration of the nipple bore. The soft metal of the tube, at least theoretically, cold flows into the grooves during swaging, such that a gas-tight seal is created between the fitting and the tube.
In practice, however, it has been found that such fittings, when swaged as described do not maintain a gastight seal, particularly over a period of time. Such a fitting is usually provided with a threaded collar or nut immediately adjacent the nipple for connecting it to another fitting on a meter or pressure regulator. The collar normally turns independently of the fitting and threadably connects with the threaded sleeve on the other fitting. When turned by a wrench, it thus forces the grooved nipple into its matingly-configured receptacle, as described above.
Because the connector nut is in direct contact (abutment) with one end of the nipple, torque is applied to the latter when the nut or collar is turned. Accordingly, there is a strong tendency for the nipple to rotate on the tube with the nut, and this destroys the gas-tight seal since the tight contact between the fitting and the tube is weakened, the structural strength of the swaged material causing adhesion between the grooved surfaces of the bore and outside surfaces of the tube being insufficient to resist the turning of the fitting on the tube due to the torque applied to the nut. When the gas-tight seal is broken, of course, gas escapes from the system around the fitting, creating a fire and explosion hazard.